Agent job failed at night but succeeded during the day

Hi,

This is a new ssis package in sql 2008R2.

It has 3 steps.

Step 1, exec sql to truncate 1 staging table.

Step 2. Data flow task to copy txt file from sql server's local drive into the db.

Step 3. exec sql update.

It runs fine in BIRD, from SSMS' manual execution, or scheduled as job to run in work hours. However, nightly job failed with following error(=cannot open source file on step 2):

Code: 0xC020200E     Source: Data Flow Task Flat File Source [1]    Description: Cannot open the datafile "D:\FilesIn\Inputlist.txt". End Error  Error: 2013-05-20 02:15:01.22    Code: 0xC004701A     Source: Data Flow Task SSIS.Pipeline    Description: component "Flat File Source" (1) failed the pre-execute phase and returned error code 0xC020200E. End Error  DTExec: The package execution returned DTSER_FAILURE (1). Started:  2:15:00 AM  Finished: 2:15:01 AM Elapsed:  0.499 seconds.  The package execution failed. 

The problem is I cannot find any reason. Any suggestion?

Thanks!